Carbonic anhydrase activity of chick osteoclasts is increased by parathyroid hormone

Embryonic chick osteoclasts were harvested, cultured for 18 h, and separated from erythrocytes, and carbonic anhydrase (CA) activity was assayed. Cultures contained 75-85% osteoclasts by acid phosphatase stain. CA activity was also determined after exposure to parathyroid hormone (PTH). Two methods of measurement were used: a delta pH method, which detects the generation of hydrogen ion by carbonic anhydrase, and a mass spectrometric method, which follows the disappearance of 18O-enriched CO2. Unstimulated osteoclasts showed CA activity that was one-third that of erythrocytes but was comparable with macrophages and chondrocytes. Osteoclasts exposed to PTH showed a twofold increase in activity, which occurred after 30 min. Ethoxzolamide (10(-8) M) inhibited enzyme activity by 90%. Treatment of osteoclasts with calcitonin did not prevent the PTH-associated increase in CA activity. The doubling of CA activity with PTH stimulation supports the hypothesis that osteoclastic CA is intimately involved in bone resorption.